Field of the Invention
The present invention relates generally to the field of vibration testing of objects such as satellites, instrumentation or any other object whose reliability in operation may be evaluated using high intensity vibration testing. Specifically, the present invention relates to the application of techniques developed for direct field acoustic testing systems to the performance of vibration testing to a predetermined specification in a semi-reverberant enclosure.
Background of the Invention
The specification of co-pending U.S. application Ser. No. 13/117,870, filed May 27, 2011 titled Direct Field Acoustic Testing System and Method (hereinafter “the '870 application”) is incorporated by reference herein. As discussed in the '870 application, in the field of Direct Field Acoustic Testing (DFAT) it is generally desirable to obtain an acoustic field having a uniform spectral content and low coherence throughout the space around the Unit Under Test (UUT). As demonstrated in the '870 application excellent spectral uniformity and low coherence was obtained at the control microphone locations through the use of a Multiple-Input-Multiple-Output (MIMO) arrangement incorporating multiple groups of independently controllable acoustic transducers. As discussed in U.S. Provisional Application No. 61/552,081 and International Application No. PCT/US12/62255, both titled Drive Signal Distribution for Direct Field Acoustic Testing, each of which is incorporated by reference herein in its entirety, improved spectral uniformity at non-control microphone locations was obtained by distribution of combinations of drive signals to the groups of independently controllable acoustic transducers. However, to achieve the high acoustic levels required for many spacecraft tests very large arrays of acoustic transducers and associated amplification delivering substantial electrical input power are required. Substantial cost and effort is required to transport, deploy and teardown said equipment and the high levels of input power increase the risk of failure. Additionally, it is difficult to scale down the amount of equipment required for testing small objects such as components leading to a relatively high cost for direct field acoustic testing of such smaller items. Previously attempts have been made to develop efficient methods of testing smaller objects using Single-Input-Single-Output (SISO) control architecture such as described in “Small Direct Field Acoustic Noise Test Facility” Saggini, et al. presented at the 26th Aerospace Testing Seminar. March 2011. This method utilized a large number of control microphones and a large number of acoustic sources installed on the interior walls of an enclosure. Inputs from the microphones were averaged and equalization coefficients calculated on octave band-widths to obtain the desired acoustic spectrum. Real time adjustments were made during testing with a SISO control architecture. This method was reasonably successful in obtaining a uniform acoustic spectrum on a full octave bandwidth basis. However, as is well known to those with skill in the art the narrow band phenomena of enclosure resonances, standing waves and wave interference patterns are the greatest problem for field uniformity in an enclosure. No narrow band spectral data is given and no coherence data is given in the Saggini paper. However, as discussed in the '870 application SISO methods do not produce good narrow band uniformity and have no ability to control coherence. Accordingly, it is desirable to provide a device and method for achieving the required acoustic levels and acoustic field characteristics with less equipment, less electrical input power and in a manner that can cost efficiently accommodate acoustic testing of smaller objects.